1. Field of the Invention
The invention relates to rotating computer disc memories wherein magnetic material is applied to a disc surface and magnetic read/write transducers are brought into close proximity with said surface so that information may be stored and retrieved from the magnetic disc memory. The invention proposes a method whereby the surface roughness of the surface of magnetic rotating discs may be improved and/or manufacturing economies enhanced through the use of a specially defined intermediate layer interposed between the magnetic disc substrate and the layer containing the magnetically sensitive material.
2. Description of the Prior Art
The trend over the last decade for computer applications has been toward a dramatic increase in the need for storage capability. In many of these applications, disc storage has been chosen as the means by which customers keep large amounts of information "on line." Due to the increased usage of disc storage, it is most advantageous from a cost standpoint (cost to the manufacturer as well as cost to the ultimate user) to store as much information as possible within a given area of magnetic discs. Thus, the density of magnetically recorded information has rapidly increased in recent years, such density increases often going hand in hand with magnetic read/write transducers operating closer to the magnetic media. The end result of this perceived technological trend has been a requirement that magnetic storage discs as used in computer rotating memories be smoother so that air-bearing read/write transducers can fly closer to the media during their magnetic recording and reading operations as well as starting and stopping in contact with said magnetic recording media.
The trends in the development of magnetic recording discs have proceeded along two distinct lines:
(1) Particulate discs in which the magnetic recording media consists of finely dispersed magnetically sensitive particles suspended in a polymeric binder applied directly to the substrate which has been composed most often of an aluminum alloy, and
(2) Thin film discs in which the magnetic recording media is a thin continuous film applied directly to the substrate which has often been composed of an aluminum alloy but glass and other materials have been utilized. In the case of particulate discs, it has often been the practice to coat the aluminum substrates by a process known as spin coating not unlike that taught in U.S. Pat. No. 3,198,657 to Kimball, et al. The finely dispersed magnetically-sensitive particles are applied within liquid resin which is carefully controlled in viscosity to create a more or less uniform layer once cured. However, the as-cured disc (that is, with the resin crosslinked at an elevated temperature and all solvents evaporated) is not smooth enough or thin enough to properly permit magnetic recording. The cured disc must subsequently be polished and/or burnished to obtain an ultimately smooth surface for magnetic recording. While the polymeric binder suspending the magnetic particles above the substrate is somewhat forgiving of defects and irregularities in the substrate finish, it is a practical reality that even after polishing and/or burnishing of the particulate disc coating, substrate defects are reflected in the ultimate surface topography of the particulate disc. Thus, while the processing operations attendant to the polishing of particulate media have a significant bearing on the ultimate surface finish, the substrate roughness cannot be overlooked as a prime contributor to the ultimate smoothness of any finished magnetic recording disc.
In the case of thin film media, the need for highly smooth, defect-free substrates for the application of thin magnetic films is greatly augmented over the case of particulate discs. As the thickness of thin magnetic films can be less than five microinches (1/8th micron) and as the processing techniques used for the application of thin magnetic films tend to exactly replicate the base structure, thin magnetic film discs are totally unforgiving of surface topographic irregularities. It is known in the prior art that metallic substrates (e.g., aluminum) of magnetic discs can be finished to an excellent surface finish by a variety of mechanical techniques. Such techniques include (1) lapping, and (2) turning of the substrates using a diamond tool bit. As a further processing refinement, aluminum substrates have been lathe-turned in the prior art using a diamond tool bit where the spindle supporting the disc substrate was of an air bearing type. While such refinements do indeed make smoother substrates, the cost of processing can be prohibitive.
Another method for improving the surface of the substrate is to coat the substrate with a layer of a suitable material which can be more easily and thus less expensively rendered smoother than the base metallic substrate. U.S. Pat. No. 3,523,824 to Powers, et al. teaches the application of a thin layer of plastic upon the metal substrate so as to provide a layer that is smooth and insulating. While it is clear from Powers, et al. that the importance of substrate smoothness is paramount, Powers, et al. is directed toward solving an underlying problem which is seen as "the formation of effective metal-to-plastic bonding sites in a significant concentration upon the surface of an ultra-smooth plastic substrate without impairing the smoothness of the substrate surface." Powers, et al. does not explain any particular process or method for achieving an ultra-smooth surface upon the intermediate plastic layer and does not define any physical constraints upon the subject intermediate layer. U.S. Pat. No. 4,046,932 to Hartmann, et al. describes an improved magnetic recording disc. Hartmann, et al. again recognizes the need for extremely smooth substrates and, in fact, suggests that an intermediate layer of coating resin may be used to enhance the effective surface smoothness of the substrate. However, the method and processes that might be used to achieve and enhance surface smoothness of the intermediate layer are not discussed.
U.S. Pat. No. 3,959,553 to Hartmann, et al. specifically deals with an improved magnetic recording disc of enhanced surface smoothness by utilizing an intermediate layer composed of a resin binder containing hard, non-magnetic pigment with a Mohs' hardness of at least seven (7). By loading the polymeric or resin binder with extremely hard particles, the intermediate layer obtains mechanical properties of enhanced hardness allowing grinding and polishing of the surface of the baked intermediate layer to achieve the desired slight surface roughness. The additional processing steps of baking and grinding and/or polishing of the intermediate layer are requirements to practice their invention.